Question

Everyone knows the classic model of a process listening for connections on a socket and forking a new process to handle each new connection. Normal practice is for the parent process to immediately call close on the newly created socket, decrementing the handle count so that only the child has a handle to the new socket.

I’ve read that the only difference between a process and a thread in Linux is that threads share the same memory. In this case I’m assuming spawning a new thread to handle a new connection also duplicates file descriptors and would also require the ‘parent’ thread to close it’s copy of the socket?

Answer 1

No. Threads share the same memory, so they share the same variables. If you close socket in parent thread, it will be also closed in child thread.

EDIT:

• man fork: The child inherits copies of the parent’s set of open file descriptors.

• man pthreads: threads share a range of other attributes (i.e., these attributes are process-wide rather than per-thread): […] open file descriptors

And some code:

#include <cstring>
#include <iostream>
using namespace std;

#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <unistd.h>

// global variable
int fd = -1;

void * threadProc(void * param) {
    cout << "thread: begin" << endl;
    sleep(2);
    int rc = close(fd);
    if (rc == -1) {
        int errsv = errno;
        cout << "thread: close() failed: " << strerror(errsv) << endl;
    }
    else {
        cout << "thread: file is closed" << endl;
    }
    cout << "thread: end" << endl;
}

int main() {
    int rc = open("/etc/passwd", O_RDONLY);
    fd = rc;

    pthread_t threadId;
    rc = pthread_create(&threadId, NULL, &threadProc, NULL);

    sleep(1);

    rc = close(fd);
    if (rc == -1) {
        int errsv = errno;
        cout << "main: close() failed: " << strerror(errsv) << endl;
        return 0;
    }
    else {
        cout << "main: file is closed" << endl;
    }

    sleep(2);
}

Output is:

thread: begin
main: file is closed
thread: close() failed: Bad file descriptor
thread: end